The present invention relates generally to the field of tire recapping, and more particularly, to the area of producing recapped tires using pre-cured tread sections.
Typically, tires have been recapped or retreaded by grinding down worn tires to their casings and then by replacing the lost tread using one of several different processes. One such process aligns the casing within a mold having inscribed on the interior surfaces thereof an appropriate tread design. Encased within this mold around the old tire casing is a mass of uncured material which is then heated to a cured state within the particular tire mold adapted for this purpose. Although this process has performed adequately in the past, it has suffered from the drawback that each tire size and tread design has required separate molds, typically costing thousands of dollars each.
More recently, a process has been used to recap or retread tires in which pre-cured tread of any given design is cemented to the prepared tire casing. This process is accomplished as follows: First, the tire casing to be processed is prepared by grinding or buffing off any old tread remaining on the casing. Cement is then applied and a layer of uncured cushion stock is applied to the cemented casing. An additional layer of cement is applied over the uncured cushion stock and a pre-cured tread layer is then applied over the cemented cushion stock. Finally, the entire composite uncured retread is inserted into a protective covering or envelope for encasing the exterior surfaces of the uncured retread during the curing of that retread. The purpose of this protective cover is to prevent air and/or steam used during the curing process from coming into contact with the cement and uncured cushion stock during the curing process. A further purpose of the protective cover is to compress the various components described above into tight engagement with the casing so that a suitable product is produced during curing. Typically, after the protective cover or envelope is placed around the composite uncured retread, the encased retread is then put into a curing oven at a temperature of 250.degree. to 325.degree. F. in an atmospheric pressure of 50 to 150 pounds per square inch. Following the curing process, the encased retread is removed from the curing oven and the protective cover is removed to produce the finished retread.
Typically, several types of retread covers have been used satisfactorily in the above described process. One such prior art product used in this process has been constructed from elastomeric calendar sheet. Two sheets of the material to be utilized in constructing this cover are cut in identically sized rings and then seamed together around the outer edges in order to produce a tire cover of the desired shape. Since the sheet goods are necessarily seamed together prior to the curing of the sheet stock, certain problems have arisen in producing a reliable seam along the outer edges thereof while preventing the more interior portions of the calendar sheets from adhering to their adjacent layer. It should be noted that the curing process which is referred to in relation to the protective tire cover or envelope is not the same curing process which is referred to in producing a cure between the pre-cured tread, cement, cushion stock and tire casing during the retreading process, but rather is the curing process which is necessary to cure the calendar sheet itself and to produce the cured seam between layers of the sheeting prior to using the protective cover or envelope in the retread manufacturing process.
Basically, there have been several approaches to satisfactorily curing the calendar sheet to produce a good, even radial seam around the outer edge while preventing the inner portions of the layers from adhering to each other. One method utilizes a dry lubricant powder which is introduced between the portions which are not to be joined while maintaining the seaming surfaces free from that powder so that they may be joined during the curing process. The cover is then cured in a flat configuration. This particular approach has the drawback of producing weak seams when powder is inadvertently introduced between the seaming surfaces. Irregular covers are produced in those instances where the inner portions of the sheeting are inadequately coated with the lubricant powder and therefore the layers adhere to each other during the curing process. It is very difficult to produce a strong, precise circular seam around the outermost edge of the calendar goods to be joined. Finally, cold checking results and air blisters further reduce the durability of the completed product.
In order to overcome some of the disadvantages of the calendar sheet covers, an alternate process has been developed wherein a butt seamed tube, very similar to a tire tube, is slit along its innermost radial periphery and is then used in the manner described above. This particular type of cover, however, has the drawback that it too not properly conform to the outer surfaces of a composite uncured retread, nor does the substitution of a tubular design substantially increase the efficiency of the retreading process over that heretofore obtained by the calendar sheet covers. Furthermore, by reason of the method of manufacture of both the calendar sheet and tubular covers, there can be no variation in the thickness of the covers, nor is any tear resistance imparted to the cover during the manufacturing and/or curing process. To the contrary, the slitting of a tube in the manner described above tends to result in the nicking of surfaces of the tube which then, during the utilization of the tube cover in the curing process, results in the tearing of that cover after relatively few uses thereof. Similarly, each of these covers has a tendency to slit along the seam and deterioration of the covers described herein usually limits their usage to between one and ten retread curings.